SLC6A3 and dementia: Individuals>=50 years of age without dementia (n = 103), divided into 5 groups (Group 1: amyloid-positive and abnormal DAT-SPECT, Group 2: amyloid-negative and abnormal DAT-SPECT, Group 3: amyloid-positive and normal DAT-SPECT, Group 4: mild cognitive impairment unlikely due to AD with normal DAT-SPECT, Group 5:cognitively normal with amyloid-negative and normal DAT-SPECT)